It often becomes desirable when producing an oil well, to treat the well to enhance and increase flow, such as by applying an acidic solution to the producing formation under pressure or by applying a hydraulic solution to the formation under extremely high pressure in order to fracture the formation. In the past, it was necessary to "kill" the well by pumping a fluid, typically mud or water, into the well until sufficient hydrostatic pressure was obtained to overcome the pressure of the formation and prevent the blowing out of fluids from the well. The wellhead was then removed, and the necessary treating apparatus tied into the production tubing. After treatment, the well then had to be swabbed to re-institute production. This cumbersome process is superceded by a wellhead isolation system disclosed in U.S. Pat. No. 3,830,304 entitled "Wellhead Isolation Tool and Method of Use Thereof," issued to Alonzo E. Cummins and assigned to Halliburton Company, Duncan, Okla. The apparatus described therein provides means for directly communicating with production tubing without the removal of the wellhead, killing the well, or swabbing the well after treatment. This is accomplished by providing a hollow high pressure mandrel slidably engaged within a high pressure casing, the casing being adapted for sealing contact with the wellhead and the mandrel being adapted for selective sealing engagement with the upper end portion of the production tubing below the wellhead. The mandrel can be extended or retracted for engagement or disengagement with the production tubing without necessitating the removal of the wellhead. The treating fluids can then be supplied to the well through the mandrel directly into the production tubing of the well without subjecting the wellhead to the high pressures in the mandrel and production tubing. When the mandrel is extended in order to supply fluids to the well, the end of the mandrel is inserted within the production tubing. In order to maintain high pressure and flow rates when pumping sand-laden fracturing fluids through the previously described equipment into the production tubing, it is necessary to provide a seal means between the outer surface of the mandrel and the interior of the production tubing. Without such seal means, the wellhead is subjected to the high pressure of the treating fluid, which may cause damage to or even destroy the wellhead. Several methods of sealing the annulus between the outer surface of the mandrel and the inner surface of the production tubing have been employed.
U.S. Pat. No. 3,830,304, issued to Alonzo E. Cummins and referred to previously, discloses a seal collar at the lower end of the high pressure mandrel having an outwardly-flared sealing cup disposed about the end thereof. The sealing means is mechanically retained on the seal collar by axial pressure generated during assembly of the mandrel and collar. Above the sealing cup on the seal collar is located a secondary sealing means comprising an elastomeric ring, which is also mechanically retained by compression on the collar.
U.S. Pat. No. 4,023,814, issued to Charles A. Pitts and assigned to the Dow Chemical Company discloses a "packer cup assembly" similar to the seal collar of the above mentioned patent. An elastomeric sleeve extends downwardly beyond the end of the mandrel, and is flared outwardly to provide a seal between the mandrel and the interior wall of the production tubing when pressure is applied. The sleeve is bonded by bonding material to the metal of the mandrel.
U.S. Pat. No. 4,111,261, entitled "Wellhead Isolation Tool," issued to Owen Norman Oliver and assigned to Halliburton Company, Duncan, Oklahoma, discloses a seal collar spaced from the end of the high pressure mandrel, with an outwardly-flared sealing cup disposed about it. An elastomeric ring is provided above the cup as a secondary seal means. Both seal means are retained in place on the seal collar by axial compression generated by the assembly of the mandrel. A second sealing configuration is also described. In lieu of a sealing cup disposed about a seal collar, the cup, of an elastomeric material, is located at the extreme end of the mandrel and is molded and bonded thereto.
The disadvantages of the prior art apparatus are that the seal collars are difficult to insert in the well without damage to the cups, that known seal means utilizing a seal means with or without an additional gasket or ring does not provide an adequate seal, that seal means located at the very end of the higher pressure mandrel are susceptible to erosion and deterioration as a result of the fluid being pumped into the production tubing, and that the seal means of the prior art configuration necessitate relatively costly and complex manufacturing procedures with rigid quality control to assure an acceptable final product. In addition, the thickness of the prior art seals severely limits the inner diameter of the seal collar, promoting erosion, when fluid is pumped at high flow rates. Finally, the seal collars of the prior art must be discarded after a single use due to the aforementioned deterioration and the inability to replace the seals on the seal collars.
In contrast, the present invention overcomes the disadvantages and limitations of the prior art by providing an easily insertable seal collar, preferably referred to as a "guide nose," having a redundant, simple and easily replaceable effective sealing means, which can be assembled without bonding, forming or mechanical restraint of the sealing means on the guide nose. The present invention contemplates a guide nose for attachment to the lower end of a high pressure mandrel, the guide nose having a bore therethrough which has a diameter at its upper end equal to that of the mandrel bore, and a diameter at the lower end only slightly less than the interior diameter of the upper end portion of the oil well production tubing, the bore diverging between the upper and lower ends of the guide nose to provide a gradually enlarging bore therebetween. The guide nose possesses two circumferential grooves on its exterior, said grooves being spaced from the ends of the guide nose and from each other, for the installation of sealing means thereon. Each groove on said guide nose is double-stepped, that is to say that at each groove location the exterior of the guide nose has two different outside radii axially spaced from one another, with a third, different lesser outside radius therebetween, thus defining a groove. The higher of the two shoulders of each groove is at the upper edge of the groove, as the tool is oriented in the well, and the other is at the lower edge. Each sealing means installed at a groove location comprises an elastomeric sleeve in the shape of an inverted cup, as oriented in the well. The wall of each seal at its upper end is of a thickness substantially equal to the difference in radius between the bottom of a groove and the higher shoulder, the sleeve having an inner diameter substantially equal to that of the outside diameter of the guide nose at the location of the grooves; this configuration is maintained for an axial distance substantially equal to the width of the groove on the guide nose. The wall thickness of the remainder of each seal is substantially equal to the difference in radius between the higher and lower shoulders of a groove. This latter portion of the seal wall, however, is flared at an angle divergently outward from the axis of the seal, being formed in such a shape so as to contact the interior wall of the production tubing when the guide nose is inserted. The length of the flared or skirt portion of each seal is substantially equal to the axial length of a recess axially below each groove on the guide nose, the diameter of each recess being equal to that of the lower shoulders of the grooves. This permits the flared portion of the seal to be collapsed toward the axis of the guide nose during insertion into the production tubing, and aids the direction of high pressure fluid under the flared portion of the seal when the tubing is pressurized. The sealing means are installed on the diffuser at the groove locations, with the flared portion pointing downward, the thicker portion of each seal wall being retained in a groove by the elastomeric properties of the seal material and the mating configuration of the thicker portion of the seal wall and the groove. Upon insertion of the guide nose into the production tubing, and application of pressure to the well, the flared portions of the seal walls are pressed against the interior wall of the production tubing, effecting a fluid-tight seal.